This invention relates to a fluid signal transmitting apparatus and particularly for transmission of a pilot or control fluid signal for controlling an output fluid signal.
Fluid control and operating systems are employed in various applications. In many systems, a relatively low level lever fluid signal constitutes an input signal to a fluid control system. The relatively low level signal is coupled through a suitable amplifier and/or relay means to generate an appropriate output pressure signal for operating a suitable load device, which may require a relatively high flow capacity. In a direct acting system, the output signal directly increases and decreases with the control signal level. A direct or reverse acting function between the output signal and the pilot or control signal may be developed. A reverse acting system is used in order to maintain compatability with the actuator or load characteristic. In a reverse acting system, the output signal decreases as the pilot or control signal increases and conversely increases as the pilot signal decreases. Various fluid amplifying devices and signaling systems may be incorporated into either system for amplifying of the control signal and/or the output pressure signal. For example, a typical method of inverting the pilot signal uses a reversing relay unit in which a plurality of stacked diaphragms define interrelated input and output chambers. The pilot or control signal and the output signal are applied to opposing chambers and a force balance condition on the several stacked diaphragms results in control of the output signal. In a particularly practical system, three diaphragms are mounted in stacked relationship with the pilot pressure applied between the middle and upper diaphragms. The output pressure is derived from the bottom chamber. A modulating output valve in the bottom chamber is coupled to the adjacent diaphragm unit and positioned in response to the relative balance of the pilot and output pressures to vary the output pressure in a predetermined manner with respect to the pilot pressure. The upper chamber constitutes a reference chamber, which may be provided with a spring or other preload means for adjusting the output set point of the relay. Although stacked diaphragm-type relays are widely used, such devices have certain known limitations, particularly with regard to accuracy. Thus, the output signal amplification is directly related to the ratio of the respective diaphragm areas. Even a small difference in the effective areas of the diaphragms may often create significant inaccuracy in the output ratio. Further, the effective area may change with pressure level and as a result the linearity of the device is adversely affected. Many such relays also have significant hysteresis which prevents precise response during increasing and decreasing signal levels. A particularly satisfactory booster relay which eliminates some of the problems of hysteresis is shown in copending patent application of Thomas M. Holloway et al, Ser. No. 964,068 filed Nov. 27, 1978. As more fully disclosed therein, hysteresis can be significantly reduced by providing spaced exhaust and supply valve units in the output chamber which seat independently in response to movement of a diaphragm, and with the diaphragm moving into sealing engagement with the exhaust valve port.
There is therefore a demand in the art of fluid signal relays or transmitting apparatus which provides highly accurate tracking and a linear output response with minimal or essentially no hysterisis, while maintaining a rugged, inexpensive and readily commercially produced construction.